A low-temperature fired ceramic circuit board has such advantages as being excellent in high frequency property and heat dissipation properties, being able to use a low-loss conductor, and being applicable to a base for an airtight sealed package. Thus, low-temperature fired ceramic circuit boards have been in practical use as a base mounted on an electronic device or a constitutional member of an electronic device package. Low-temperature fired ceramic circuit boards are multilayer boards in which circuit wiring is incorporated, the circuit wiring being obtained by forming a circuit pattern with a conductor paste on a green sheet produced from a powder for a green sheet containing a glass powder and an alumina powder, then laminating a plurality of the green sheets to form an integrated laminate, and firing the laminate at low temperatures. Ceramics used for the green sheet can be fired at 1000° C. or lower, and hence the ceramics are called “low-temperature fired ceramics” as a comparative expression to ceramics such as alumina whose firing temperature is as high as 1500° C. or higher.
In a high-temperature fired ceramic circuit board using an alumina green sheet, etc., a high-melting-point, high-resistance metal such as W or Mo is used as a conductor material. Accordingly, the board has high conductor resistance, and hence the board has such a problem that circuit loss is large. In addition, in a process of manufacturing high-temperature fired ceramic circuit boards, a reduction atmosphere is required, resulting in a complicated manufacturing process and a complicated manufacturing facility.
In contrast, in the low-temperature fired ceramic circuit board, a low-resistance metal such as Au or Ag is used as a conductor material, and hence the board is low in circuit loss. Here, when the low-temperature fired ceramic circuit board is manufactured, the firing temperature needs to be lower than the melting points of Au and Ag. Thus, a green sheet is produced by using a powder for a green sheet containing a glass powder containing, as a main component, glass that softens at lower than 900° C. to form a dense state and an alumina powder. Au and Ag are easy to use because they can be fired in air, and in particular, because Ag is inexpensive among low-resistance metals. Accordingly, a conductor paste containing Ag as a main component has been frequently used. However, Ag has a feature that it is apt to diffuse into the glass component of a green sheet.
In recent years, in the information communications field, the frequency bands of radio waves used have become higher owing to the increased amount of information and diversified communication systems, and microwave and millimeter-wave bands have come to be used. As the frequency of the radio waves used becomes higher, the more the radio waves transform into heat in a circuit, that is, transmission loss becomes higher. Thus, users who desire higher performance, lower power consumption, and higher output ability of communication equipment require a decrease in transmission loss in high-frequency bands. Factors that greatly influence transmission loss in the high-frequency bands include the dielectric properties of a ceramic board and the electric resistance of a conductor. There is little room for improvement in the conductor because Ag, which has the smallest electric resistance comparable to that of Cu, is already used in the main, and hence improving the dielectric properties of a ceramic board is important. In particular, as the frequencies become higher, the contribution ratio of dielectric properties become higher, so from the viewpoint of reducing transmission loss in high-frequency bands, a ceramic board excellent in dielectric properties (that is, low in relative dielectric constant ∈r and dielectric loss tangent tan δ) must be used. Factors influencing the dielectric properties of the ceramic board include the dielectric properties of a glass component. However, there is a problem in that compatibility between the dielectric properties of the ceramic board and low-temperature firing is generally difficult.
A known conventional green sheet that provides a ceramic board is, for example, one using a powder for a green sheet containing 12 to 59.6% by weight of alumina, 18 to 69.6% by weight of borosilicate-based glass, 1 to 40% by weight of anorthite crystals, and 1 to 5% by weight of celsian crystals (see Patent Document 1, for example). The green sheet can be fired at low temperatures regardless of firing atmospheres and can provide a ceramic board with a low relative dielectric constant ∈r and excellent mechanical strength.
In addition, compositions of aluminosilicate-based glass low in relative dielectric constant ∈r and dielectric loss tangent tans are also known (see Patent Document 2, for example).
Patent Document 1: JP 6-305770 A
Patent Document 2: JP 11-292567 A